Valves are used to control a flow of fluid through a fluid flow apparatus, and the simplest valves are simple on-off valves which either permit or prevent fluid flow through the valve. However, more sophisticated control valves are available which are able to regulate the fluid flow through the valve in a non-binary manner.
Ball valves are simple control valves; a two-way ball valve typically comprises a ball having a cylindrical bore therethrough. The valve ball can be rotated via a valve stem, which twists the bore out of alignment with an inlet and outlet of the valve. This chokes the flow through the ball valve, and therefore a flow out of the outlet of the valve can be controlled.
Unfortunately, the nature of the rotation of the valve ball is such that the overlap between the bore and the inlet and outlet is non-linear with respect to the degree of rotation of the valve stein. As such, precise control of the flow through a ball valve is not possible without downstream measurement of the flow.